Screened Porch End Wall Question

[XR250]

Attached is a typical end wall of a screened porch that Arch's send me all the time. Since I do not have a continuous king post, I typically use a 5 1/2" thick solid girt running the width of the porch to brace the bottom of the king post for out-of-plane stability.(typically, these are 18 - 20ft wide.) Most contractors would prefer me to use a 3 1/2" thick member or nothing special at all as it makes their boxing more consistent and is cheaper. I see these built without any consideration to any out-of-plane bracing and have yet to witness an issue in my 25 years of engineering. (Let's not discuss lateral in-plane stability at this point.)
I prefer they use a solid 6x6 king post from the floor to the ridge, but many Arch's do not like the way it looks. Does anyone have any further insight or alternative facts on this matter?

Thanks!

 

[msquared48]

Install two "queen" posts directly above the central two columns to the rafters above, and design the rafters to support the ridge beam, hanging the ridge beam from the rafters with an inverted hanger.

Note: I'm trying to invent new terminology here that will confuse your Architects.

Mike McCann, PE, SE (WA)

 

[XR250]

I have done that in the past. I try to avoid it as I have had issues with it getting built correctly due to the amount of detailing. Might give it another shot.
Thanks!

 

[HouseBoy]

You show two 6x6's as the intermediate posts. Why can't the girt/beam be 5 1/2" wide?
I HAVE seen problems with this as built and I can't let it go without something that is capable of spanning horizontally out to out. Usually I want the girt/beam to be a solid piece (glu-lam)
Sometimes I use a steel flitch (placed flat) or plates made of LVL's flat, ripped to 5 1/2" and built up if needed.

Usually I'm not as "strict" on the out of plane deflection calc (especially if it is screen, seems like L/180 is plenty) but I need to have something that I know is adequate for stress.

18-20 ft is far! I seldom have two big triangles of screen above the beam/girt. Usually I can convince all to frame the two side posts full height.

I hate this condition and I know what you mean about having "yet to witness an issue" still, you only need to have an issue once...

 

[msquared48]

The "Queen" posts should be continuous though (floor to rafter) so as not to create a knuckle joint for the window wall. Forgot to mention that little fact.

Mike McCann, PE, SE (WA)

 

[KootK]

Sketchy moment connection between glulam and king post?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

XR250, I think this is an under thought about condition. Have you calculated the strength / stiffness requirements on the beam to provide bracing for the king post? Does it need to be 5 1/2" to meet the bracing requirements?

 

[XR250]

You show two 6x6's as the intermediate posts. Why can't the girt/beam be 5 1/2" wide?

Not sure. Something about their boxing. Al I know is that they hate it.
I like the two queen posts full height (no knuckle joint per Msqaured)as well with the girt just acting as a show beam between the two.

Sketchy moment connection between glulam and king post?

I was wondering about that but I cannot wrap my head around if it will perform any useful function. Wouldn't that just put the girt in torsion?

 

[KootK]

What is the story that we're telling about out of plane wind loading here? How much are you assuming that the screening picks up?

Wouldn't that just put the girt in torsion?

Yeah, technically it does. Unless you can moment connect the girt to the upper post somehow. Of course, your stability demand from the post always did require torsional resistance from the girt. What can I say? I'm going for gooder here rather than all the way good.

Suppose that you made the gable frame, including the girt and some doubled up rafters, into a simple truss that was able to support the glulam clearspan. Then you would no longer have a stability issue at the bottom of the king post. You'd just have to find homes for your out of plane wind loads.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

I paged through the NDS quickly, and I don't see anywhere where it specifically states the bracing strength / stiffness requirements. If someone is aware of the provision, please let me know.

If we used AISC as an example, I believe your girt would be classified as a "Nodal" brace as the two brace points are not connected together (like diagonal brace).

Therefore, I believe you would design for brace strength of Pbr = 0.01 Pr and brace stiffness of Bbr = Omega (8*Pr/Lb).

Assuming 2,000# reaction.

Pbr = 0.01 * 2,000 = 20# (this seems easy to meet)
Bbr = 2.0 * (8 * 2 kips/48 in) = 0.67 kip / in. (assuming 48" tall king post)

Assuming DF wood with 4x12 in the flat direction, the strength requirement is relatively easy to meet. Stiffness seems to cap out at about a 14'-0" span for the girt to meet the 0.67 k/in requirement. 5 1/2" wide beam appears to meet requirements for 20'-0" span.

 

[XR250]

What is the story that we're telling about out of plane wind loading here? How much are you assuming that the screening picks up?

Zero (or I have simply neglected it) Just looking at stability of the node.

Suppose that you made the gable frame, including the girt and some doubled up rafters, into a simple truss that was able to support the glulam clearspan. Then you would no longer have a stability issue at the bottom of the king post. You'd just have to find homes for your out of plane wind loads.

Yup, thought about that. It just gets a little messy at the posts as they want the "girt" to be at the same elevation as the eave headers -so the rafters would be on top of the bottom chord.

Assuming DF wood with 4x12 in the flat direction, the strength requirement is relatively easy to meet. Stiffness seems to cap out at about a 14'-0" span for the girt to meet the 0.67 k/in requirement. 5 1/2" wide beam appears to meet requirements for 20'-0" span.

That is about what I have come up with in the past - hence my 5 1/2" width
I might have to go back to msquared's idea and start doing the cantilevered doing rafter thing.

Thanks for your help.

 

[XR250]

Suppose that you made the gable frame, including the girt and some doubled up rafters, into a simple truss that was able to support the glulam clearspan. Then you would no longer have a stability issue at the bottom of the king post. You'd just have to find homes for your out of plane wind loads.

Yup, thought about that. It just gets a little messy at the posts as they want the "girt" to be at the same elevation as the eave headers -so the rafters would be on top of the bottom chord.

Actually, I could use some diagonal straps at the heel if the beam was the same width as the dbl. rafters. Just got to sell them on the looks if it ain't cladded.

 

[HouseBoy]

Something seems a bit arbitrary here...(not saying it's wrong, inappropriate or uncommon, just sayin'...).

Seems like there are (at least) 3 "sources" for a lateral load that could/should reasonably be considered here:
1. A force to provide stability to the girt/beam (due to the vertical load)
2. Out of plane wind load (I thought ASCE-7 says screens pick up 100% of the wind)
3. Something to account for incidental loading on the interior columns (even if it is just a 200# or 50 plf handrail load)

Are we squinting too hard on this one in order to make it look OK?

I've done the end rafters as a truss thing before (to support the ridge). There was a very nice article in Fine Home Building 20 or 25 years ago about that strategy.
I've also uses C5 turned flat to make the horizontal span.

I see this condition often enough and I'm usually "the bad guy" so... very interested in the thoughts of others.

 

[EngineeringEric]

So are diagonal braces along the span from 1/3 points of the girt kicking into the roof diap. our of the question? I try to specify a girt as well if i have to (no ceiling), or a ceiling system, or kickers.

A second problem i have with something like this: If it is all screen or screen and posts then you have no shearwalls or lateral system, so we are relying on a hip roof to act as a cantilevered diaphragm, i rarely see any substantial chord connection back to the main house or proper shear transfer from the sheathing.

Honestly, i think these do fail when exposed to full design load (not common to see full design wind) and there are multiple redundancies that we cannot model or calculate. spring-hinges, load sharing, wood's large safety factor, duration factors, weak axis bending, shear transfer of screen material ()

 

[XR250]

So are diagonal braces along the span from 1/3 points of the girt kicking into the roof diap. our of the question? I try to specify a girt as well if i have to (no ceiling), or a ceiling system, or kickers.

I have never gotten approval for that.

A second problem i have with something like this: If it is all screen or screen and posts then you have no shearwalls or lateral system, so we are relying on a hip roof to act as a cantilevered diaphragm, i rarely see any substantial chord connection back to the main house or proper shear transfer from the sheathing.

Yes, but that is a subject for another thread. I typically just specify lots of nails on the overbuild and hope for the best. I kind of have to stay within the standard of practice in my area or I will be labeled as the "Bad Guy" as Houseboy put it.

 

[EngineeringEric]

I thought Bad Guys always finished first! I wasn't trying to delve into that other one, more of a, I don't think so many things don't work i just say its all magic!

As for the kickers, I got them once for a bigger span and when it was all painted really they weren't too noticeable and they were above window 'king' studs so it kind of blended in.

So it sounds like girts is the primary option or some fancy moment connections with a girt or magic.

 

[jdgengineer]

I do agree with Houseboy that the other elements do need to be evaluated, my answers were really academic as it related to post bracing.

We typically would have 5 1/4 PSL beams that would be wrapped framing into HSS cantilevered columns for lateral. So no matter what you do, you can say it's cheaper than that...

Then again typically the architect would want the porch vaulted and would not want to see the girt at all so we would end up with a kinked steel beam at the gable end to support a steel ridge as a wood ridge would have hung down 1/2" below the rafters or something and this would be "unacceptable".

 

[HouseBoy]

Also, I almost never see solid sawn 4x or 6x lumber (other than 4x6 and 6x6 PT posts) around Cincinnati OH unless it is for some fancy exposed timber look inside of a big family room so.. wondering how often others see solid sawn lumber for this. Don't see many glu-lams around here either. Most framers seem to like to build up using LVLs if engineered wood is needed.
(Don't meant hi-jack the thread, mainly wondering how common or open to solid sawn lumber a builder might be for this application.)

 

[EngineeringEric]

My understanding is west coast does more with larger dimensional lumber (4x8 6x10) and then the rest of us in the states are left building up widths to with (2)2x8 and (3)2x10. Please note this is based on me experience only in the northern California region and the mid-Atlantic.

I also rarely use or see Glulam, LVL is the primary. (My hijacking is officially over, sorry XR250)

 

[jdgengineer]

Not a screened in porch but common vaulted gable condition we run into.